2 Answers
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As of the July 30, 2013 patch, Mist Energy no longer exists in Spiral Knights. Crystal Energy has been renamed simply to 'Energy' and the purpose of Mist Wells has slightly changed. Here is a quote from the patch notes:

Mist Wells have been renamed 'Energy Wells' and no longer provide a 25% bonus on deposit.

Mist within Mist Wells has been converted to energy and may be withdrawn at 100 energy per day.

Energy Wells appear to retain the purpose of sharing energy with guild members.

The Mist Well is a method of donating Crystal Energy to your guild. It converts Crystal Energy deposits (with a minimum starting at 500CE) and converts it into Mist Energy with a 20% bonus. This means that if you deposit 500 Crystal Energy you will get 600 Mist Energy in return, but this is stored in the Mist Well and is available to all members of your guild.

You can of course withdraw from this source to replenish your own Mist Energy source.